1. Field of the Invention
The present invention relates to a microwave assisted thin film magnetic head that provides an assistance function to generate a circularly polarized magnetic field of a microwave band as an assistance method for writing data signals to a magnetic recording medium having a large coercive force to stabilize magnetization. The present invention also relates to a magnetic disk device using the microwave assisted thin film magnetic head.
2. Description of the Background Technology
Conventional magnetic recording media, such as hard disks, have undergone significant improvements in in-plane recording density through improvements such as miniaturization of magnetic particles and improvements in magnetic materials that configure magnetic recording layers, miniaturization of the head fabrication and the like. Furthermore, recent magnetic recording reproducing devices primarily use a perpendicular magnetic recording method that improves in-plane recording density by miniaturizing the recording bit by magnetizing the recording layer in a perpendicular direction to the surface of the magnetic recording medium. Further improvements in the in-plane recording density are expected in the future.
Meanwhile, because thermal fluctuation easily occurs in recording magnetization with miniaturization of the recording bit and magnetic particles, magnetic particles having a large anisotropic magnetic field Hk and a large coercive force Hc are preferred for use as material for the recording layer.
However, when magnetic particles with a large anisotropic magnetic field Hk are used as the material for the recording layer, the coercive force of the recording layer becomes a large value of 5 KOe or greater. Generally, a recording magnetic field of at least twice the coercive force is said to be required when performing saturation magnetic recording, which has been problematic for saturation magnetization of the recording layer with conventional magnetic head performance. In other words, the recording and erasing of data on a magnetic disk has been problematic.
In response to these technical problems, a novel recording method is proposed that uses microwave assistance. This method attempts to reduce the magnetization-inversed magnetic field in the perpendicular direction by forming a spin torque oscillator (STO) configured by multiple layers of magnetic thin film in the gap between the recording main pole of the perpendicular magnetic head and the rear end shield to induce magnetization precession movement by generating an in-plane microwave magnetic field by the self-oscillation of the STO. (U.S. Pat. No. 6,785,092)
Specifically, a field generation layer (FGL) of the STO configured from multiple layers of thin film self-oscillates at a high frequency, and the leakage magnetic field generated from the surface thereof is used as microwave assistance. The microwave assisted magnetic field generated here is known as a linearly polarized magnetic field. This assistance method is also known as a self-excited type.
Compared to this assistance method, a head device is proposed that arranges an auxiliary coil within a gap between the main pole and the rear end shield and that performs microwave assisted recording by generating a high frequency in-plane magnetic field within the gap by driving high frequency current to the auxiliary coil (Japanese Laid-Open Patent Application Publication No. 2007-299460). This assistance method is also known as an induced type. A magnetic head device that provides this type of assistance method can apply high frequency in-plane alternate-current magnetic fields in a microwave band to the recording layer of the recording medium. The effect of such assistance greatly reduces the perpendicular recording magnetic field. Therefore, the recording and erasing of data can be performed at a high-speed even with a recording layer having a large coercive force. Furthermore, the in-plane alternate-current magnetic fields generated by driving high frequency current to the auxiliary coil provide the following characteristics:
1) the frequency can be controlled on an order of ppm, and
2) the generated magnetic field is easily controlled by controlling the amplitude of the high frequency current.
The microwave assisted magnetic field generated by this method is also a linearly polarized magnetic field.
Meanwhile, analysis has advanced in recent years concerning the advantageous effects of recording properties through microwave assistance, and research has also been advancing around the effects of the intensity, polarization, frequency, and the like, of assistance magnetic fields. Furthermore, it has been found that use of a circularly polarized magnetic field is preferred for assistance, as a circularly polarized magnetic field can generate a stronger inversed magnetic field compared to a linearly polarized magnetic field (Y. Wang et. al, J. Appl. Phys. 105, 07B902 (2009)). From this perspective, the method disclosed in Japanese Laid-Open Patent Application Publication No. 2010-003353 proposes a method for generating a circularly polarized magnetic field in an induced type in which an STO is used. In other words, because a lamination layer plane 25a of a layered structure body 25 proposed in Japanese Laid-Open Patent Application Publication No. 2010-003353 is tilted relative to an air bearing surface (ABS) 61s, the high frequency magnetic field Hac from the spin torque oscillator 10 is used to act as a circularly polarized magnetic field in relation to the magnetic recording layer 81 of the magnetic recording medium 80.
However, the configuration of the proposed STO has problems, such as:
1) the process is extremely complicated in that the lamination of the magnetic thin film requires a minimum of five layers;
2) magnetic film having an extremely high magnetic anisotropy is required for STO oscillation;
3) control of the oscillation frequency is difficult as such control is performed only by the current density infused into the STO and as sudden changes can occur in the oscillation frequency as a result of very small fluctuations in the current density, and
4) the control parameters a microwave magnetic field that can be generated are not clear, and the size of the generated magnetic field is also not unapparent. Although a control parameter is assumed to be current density that infuses energy, the oscillation frequency changes when adjusting current density. As a result, arbitrarily controlling both parameters is difficult.
In addition to these problems, forming the element at an incline within the gap as in the proposed configuration is extremely difficult from a process perspective which makes oscillation control even more difficult.
The present invention is conceived based on these actual circumstances and has an object to provide a specific head structure and current drive method that generates a circularly polarized magnetic field with high magnetization inversion efficiency in an induced method for microwave assistance, or in other words, that generates a circularly polarized magnetic field in an induced method for microwave assistance, which has characteristics, such as, 1) that frequency can be controlled on an order of ppm, and 2) that a generated magnetic field can easily be controlled by controlling the amplitude of high frequency current.